This invention relates generally to the field of integrated optical components for use in fiber optic communications, and more specifically to photonic switching. In particular, it pertains to integrated optical crossbar matrix switches having very low crosstalk.
An optical 2.times.2 changeover switch with two input ports and two output ports has two operational states. In a first state, known as the "bar" state, the switch is activated by applying a voltage to its electrodes, and an optical signal on a first input port passes straight through to the first output port. In a second state, known as the "cross" or "changeover" state, an optical signal on a first input port crosses from one waveguide to another to appear on a second output port, as illustrated in FIGS. 1a and 1b. Typical 2.times.2 optical changeover switches are directional couplers, .DELTA..beta. directional couplers and X-switches. Electrooptic 2.times.2 changeover switches have been extensively used to build optical crossbar matrix switches ever since photonic switching was first explored. Alferness ("Waveguide electrooptic switch arrays", IEEE J. Selected Areas in Commun., Vol. 6, pp. 1117-1130 (1988)) has given a review of the status of waveguide electrooptic switch arrays.
The rectangularly configured crossbar matrix switch was reported by Sawaki et al. (Rectangularly configured 4.times.4 Ti=LiNbO.sub.3 matrix switch with low drive voltage", IEEE J. Selected Areas in Commun., Vol. 6, pp. 1267-1272 (1988)).
FIG. 2 is a diagram of such a 4.times.4 rectangular crossbar matrix switch 120 using 2.times.2 changeover switch elements 122 (represented by boxes in FIG. 2), such as reverse .DELTA..beta. directional couplers. The 2.times.2 changeover switch elements are connected by passive integrated waveguide bends 124 (thick grey lines). The diagram of FIG. 2 also shows four input ports A, B, C, D, and four output ports A', B', C', D'. FIG. 2 also indicates which switch elements 122 should be activated (driven to the "bar" state) to set up corresponding connection paths. Under each switch element 122 are two-letter codes which indicate the connection path which results from activating that particular switch element. For example, activating the upper-left switch element (code AC') in FIG. 2 connects port A to port C'. This representational technique for connection paths is utilized in subsequent figures. In FIG. 2 and subsequent figures, some integrated waveguide bends are omitted for clarity in the drawings.
FIG. 3 shows a diagram of the waveguide pattern of such a matrix switch using directional couplers, the active portion 132 of the waveguides being shown by thicker black lines. The indication of connection paths by letter codes follows the format used for FIG. 2.
This prior art switch has several advantages: (1) it reduces the switch rank from standard 2N-1 to N (the switch matrix dimension is N.times.N); (2) all connection paths have the same number of crosspoints; (3) it is a strictly nonblocking architecture in which every connection path can be set up by activating one crosspoint and can be free of interruption. The first advantage allows either reducing the switch drive voltage or increasing the matrix dimension on a single array chip. The second advantage allows uniform optical loss for all connection paths. The third advantage offers easy control and prevention of data loss. In fact, Sawaki, et al reported a 4.times.4 matrix switch (using reverse .DELTA..beta. directional couplers), similar to that of FIG. 2, which has a drive voltage of 11 V and ar average insertion loss of 4.7 dB with deviations for different paths within .+-.0.3 dB. The signal-to-crosstalk ratio (SXR) in this architecture is given by EQU SXR=X.sub.s -L.sub.i -10 log.sub.10 (N-1), (dB) (1)
where X.sub.s and L.sub.i represent the switch element extinction ratio and the optical loss of a waveguide intersection (both in dB.) Their measurements showed X.sub.s .about.15 to .about.30 dB (average X.sub.s =24 dB) and L.sub.i .about.0.3 dB, which represent the level that current processing technology can generally achieve with tight control.
From Eq. (1) one knows that, for a 8.times.8 matrix switch with SXR=20 dB, one has to use 2.times.2 changeover switches with X.sub.2 .ltoreq.29 dB, which is somewhat difficult to achieve in an integrated optical device, especially when more than 10 switch elements are fabricated on one chip.